As a user of compressed air, at some point you are likely to see water appearing in your air distribution system. This can be a nuisance or a serious issue, depending on your application. Should you need to eliminate the problem, it is important to clearly define the results required and specify the right equipment to find the most economical solution.

Where does water in compressed air come from?

Water is present in a gaseous form in the air that is drawn into the compressor. The exact amount of water in the air is called the “humidity” of the air. Other relevant terms to know include the following:

Relative humidity - The amount of water vapor that can be held in air is dictated by the temperature of the air. Hot air can hold more water (as vapor) than cold air. Typically, atmospheric air has about 50% of its water vapor holding capacity for a given temperature. The proportion of the maximum vapor holding capacity is referred to as relative humidity.

Dew point and condensation - When air with a given relative humidity is cooled, it reaches a temperature at which it is saturated. At saturation, the relative humidity of the air is 100%, meaning the air contains as much water vapor as it can hold. Cooling the air beyond its dew point (the temperature at which the air is at 100% relative humidity), results in condensation of the water vapor.

Cooling and condensation in compressed air - Compressing air causes its temperature to rise significantly. The hotter temperature increases the air’s vapor holding capacity, which, in turn, reduces the relative humidity of the air. Compressing air also increases its dew point. This means that subsequent cooling of the air, such as with an after cooler (see below for an explanation of this technology), could cause condensation. The after cooler itself removes a significant portion of the water vapor. However, the compressed air exiting the after cooler is saturated, so any further cooling of the air results in additional condensation. It is this cooling beyond the compressed air’s dew point that produces the water you see in your compressed air distribution system.

“Drying” compressed air means removing water from it. In terms of cost, the more water a technology removes, the higher the cost of drying. But don’t forget to factor in hidden costs. For example, when you permit too much water to remain in the compressed air supply, you pay the price in the form of maintenance, corrosion, or product losses.

Here are the available drying technologies:

• After cooler — Reduces the temperature and water content of the compressed air.
• Water trap — Removes bulk water condensed by the after cooler.
• Drip leg — Controls slugs of water and oil from system upsets.
• Coalescing filter — Removes aerosol water and other liquids which bypass the water traps.
• Pressure reduction — Drying through expansion.
• Refrigeration drying — To dew points of approximately 37°F (3°C).
• Chemical dryer — Reduces dew point by approximately 50°F (10°C).
• Desiccant dryer — Drying to dew points of approximately -40°F to -100°F (-40°C to -73°C).
• Membrane dryer — Variable drying capabilities to approximately -40°F (-40°C) dew point.

In an industrial manufacturing plant, coalescing filters are
probably the most important piece of purification equipment found
in a compressed air system. They treat six of the ten main
contaminants...

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